A Talbot-Lau interferometer consists of three micro-period gratings: ‘source’, ‘beam-splitter’, and ‘analyzer’. The source and analyzer are absorption grating typically made of Au, while the beam-splitter is a thin phase grating typically made of Si or Ni. To enable differential phase-contrast (DPC) imaging of thick body parts the interferometer must work at high energy. For instance, X-ray DPC imaging for the knee can potentially be done, for which radiography is typically done at 60-65 kVp (40-45 keV mean spectral energy) and conventional CT at 80-90 kVp (55-60 keV mean energy).
In addition, an interferometer must be very sensitive to small X-ray angular changes to enable refraction imaging with acceptable dose. The sensitivity is determined by two parameters: the fringe contrast or ‘visibility’ V, and the angular resolution, W. The contrast is defined as V=(IBF−IDF)/(IBF+IDF), with IBF and IDF the ‘bright-field’ and the ‘dark-field’ intensity respectively, while W is given by the ratio between the interferometer period and the distance between the gratings. High contrast (in the ≧20% range approximately) is essential for medical DPC imaging, because the signal-to-noise ratio (SNR) in the DPC images improves rapidly with increasing contrast (e.g., as ˜V2 in DPC-CT). Good angular resolution (W≦several μ-radian) is also needed, because the X-ray refraction angles in soft tissue are in the sub μ-radian range. The requirements for high contrast and angular resolution are more critical at high X-ray energy, because the refraction angles decrease with energy as ˜1/E2.
For DPC imaging of large body parts the Talbot-Lau interferometer must have ≧20% contrast at mean spectral energies ≧40 keV, while using gratings with ≦10 μm period. This is not possible however with the conventional normal incidence Talbot-Lau interferometer, because the thickness of few micron period absorption gratings is technologically limited to ˜100 μm. To illustrate this limitation, in FIG. 1A, the computed contrast of a first Talbot order (m=1), 5 μm period interferometer designed for 55 keV mean energy, and having 100 μm thick, 50% duty-cycle Au gratings is plotted. Also plotted is the spectrum of an 80 kVp W anode tube after transmission through 2 mm Al, 75 μm Cu and 150 mm soft tissue. The maximal contrast is low and the contrast curve overlaps poorly with the tube spectrum, making for a spectrally averaged contrast of only ˜6%. For comparison, an interferometer having perfectly absorbing gratings would have ˜32% averaged contrast.
A device that enables phase contrast imaging at high X-ray energy is the glancing angle Talbot-Lau interferometer (GAI), in which the gratings have bars inclined at an angle α˜10-30° along the beam direction. The effect of inclining the gratings is to increase the effective absorber thickness from the normal incidence value t, to t/sin(α). Because the X-ray absorption increases exponentially with the thickness, this enables achieving high contrast at high energy using the existing ˜100 μm thick gratings.
The main limitation of a GAI device, such as the one described above, is that the field of view in the direction perpendicular to the grating bars is limited (vignetted) to ≦few tens of mm by the strong collimation in the inclined grating openings, as illustrated in FIG. 1B. At the same time, an FOV of up to several tens of cm is needed for CT of larger objects such as thick body parts or checked baggage. In addition, previous research shows that the optimal configuration for phase-contrast tomography (PC-CT) is with the grating bars parallel to the CT axis, as in FIG. 1B.
It would therefore be advantageous to provide a device that combines in an efficient and accurate manner multiple GAI gratings so as to make large FOV interferometric systems that will allow DPC-CT and imaging of large objects.